Since the melting point of metal nanoparticles having a particle diameter on the order of several nanometers differs dramatically from that of a bulk metal, these metal nanoparticles have conventionally been expected to be applied to electrically conductive pastes and the like which can be used for a low-temperature baking. In the past, methods involving the reduction of a metal in a solvent are known to have been used to produce such metal nanoparticles.
For example, numerous techniques have been studied as methods for synthesizing silver nanoparticles in an aqueous solution, a typical example of which is the so-called Carey-Lea sol method in which an aqueous silver nitrate solution is added to an aqueous solution of a ferrous salt and a citric acid salt. According to this method, a silver colloid dispersion can be obtained which has superior dispersion stability, a particle diameter on the order of 10 nm, and a narrow particle size distribution. By coating a dispersion containing such metal nanoparticles on a base material, and then baking at a low temperature, a film is formed which has a volume resistivity and a reflectance close to those of bulk metals, and this film can be used as an electrode. When using these metal colloid dispersions, it is known that their properties can be changed considerably by controlling the particle diameter, the particle size distribution, and the shape of the microparticles.
As an example of a method for producing such metal nanoparticles, a method for forming a nanoprism is disclosed in which silver crystals are formed by exposing a dispersion of amorphous silver particles to a light source having a wavelength of less than 700 nm (see, for example, US Patent Application, Publication No. 2003/0136223 (claim 1, FIG. 7)). According to this method, plate-like silver single crystals are formed.
In addition, a method for producing silver powder composed of hexagonal plate-like crystalline silver particles is disclosed which includes temporarily mixing a slurry containing an ammine complex of a silver salt and an ammine complex of a heavy-metal salt that functions as a habit modifier during a reduction reaction, with a solution containing potassium sulfite that is a reducing agent and a gelatin that is a protective colloid, so as to reduce the ammine complex of the silver salt, and recovering the formed silver particles (see, for example, Japanese Unexamined Patent Application, First Publication No. H11-106806 (claim 2, paragraph [0021] of specification)). In the method disclosed in Patent Document 2, hexagonal plate-like crystalline microparticles having a primary particle diameter of 5 to 10 μm are obtained.
Moreover, silver microparticles are known that are roughly plate-like particles having two principal planes and having a particle thickness of 50 nm or less and a length of a major axis of 5000 nm or less (see, for example, Japanese Unexamined Patent Application, First Publication No. 2005-105376 (claims 1 and 4)). In the method disclosed in Patent Document 3, silver microparticles are produced by stirring a solution in which at least a polymer compound, a reducing agent, and a silver salt are dissolved at a temperature of 25 to 60° C.
On the other hand, as was previously described, since the melting point of the metal nanoparticles having a particle diameter on the order of several nanometers differs dramatically from that of a bulk metal, the metal nanoparticles have conventionally been expected to be applied to the electrically conductive pastes and the like which can be used for a low-temperature baking. In order to produce such metal nanoparticles, methods have been used in the past in which a metal is reduced in a solvent like that described above.
Therefore, in order to prepare desired metal nanoparticles in the form of a uniform dispersion, it has been considered that it is necessary to use a raw material metal salt (metal compound) that dissolves in the liquid (solvent), and the number of raw materials available for that use have conventionally been limited for that reason. For example, in the case of obtaining silver nanoparticles, soluble compounds have been exclusively used, such as silver nitrate or silver perchlorate in the case of using aqueous solvents, and silver complexes in the case of using organic solvents.
However, since silver halides can be easily acquired for use as silver raw materials from the viewpoint that they are already produced in large volume as photographic raw materials, and since they can also be handled easily as solids, it would be preferable if silver nanoparticles could be produced from such insoluble silver salts. From this viewpoint, a method for producing silver nanoparticles by reducing a silver salt in a solvent has been proposed in which a silver halide is used as the silver salt, a metal coordinating compound capable of dissolving in the solvent and coordinating with silver is used as a protective agent for the silver salt serving as the raw material, and the reduction is carried out in the presence of the protective agent composed of that compound (see, for example, Japanese Unexamined Patent Application, First Publication No. 2003-253311 (paragraphs [0002] to [0005] of specification)). This method is based on the finding that by using a specific protective agent, an insoluble silver salt such as a silver halide can be effectively reduced in a solvent to produce nanoparticles, and according to this method, silver nanoparticles can be produced from an insoluble silver salt.